1. Field of the Invention
The present invention relates to a wire-cut, electric discharge machine which is adapted to eliminate the possibility of a wire leaving the path of the running system when the wire breaks.
2. Description of the Prior Art
A wire-cut, electric discharge machine performs machining of a workpiece into a desired shape by producing a discharge between a wire installed on its running system and the workpiece. The wire may sometimes break during cutting unless the tension of the wire, the feed rate and the cutting conditions are appropriate. Since the wire is given constant tension by a tension brake when the wire breaks is likely to instantly come off its running system. If this phenomenon extends to the vicinity of the wire supply reel, reinstallation of the wire on the running system is very troublesome and impairs machining efficiency.
FIGS. 1A and 1B schematically show the breakage of the wire in a conventional wire-cut, electric discharge machine. In FIGS. 1A and 1B, reference numeral 1 indicates a wire; 2 designates a wire take-up reel; 3 identifies wire feed rollers; 4 and 8 denote feed pins; 5 represents an upper guide; 6 shows a workpiece; 7 refers to a lower guide; 9 indicates a guide roller; 10 designates a tension brake; and 11 identifies a wire supply reel.
During cutting the wire 1 is installed on the running system, as shown in FIG. 1A, and subjected to fixed tension by the tension brake 10, as mentioned previously. Accordingly, when the wire 1 breaks, it comes off the running system, as depicted in FIG. 1B, and reinstallation of the wire 1 on the running system is very time-consuming, as described previously.
To avoid such a defect, the general practice in the prior art is to provide felt pads (not shown) for gripping therebetween the wire 1 in the neighborhood of each of the upper and lower guides 5 and 7 in order to stretch the wire 1 perpendicularly to the reference plane of the workpiece 6 to prevent the wire 1 from getting out of the running system when it is breaks. With the conventional method, however, it is difficult to adjust the pressure of the pads so that the tension of the travelling wire 1 is uniform over the entire range of the running system; lack of uniform tension also leads to the drawback that the wire 1 is more apt to break.